1. Technical Field of the Invention
The present invention relates to an ignition coil, for an internal combustion engine, for generating a high voltage for application to a spark plug of the internal combustion engine.
2. Description of the Related Prior Art
An ignition coil for an internal combustion engine (hereinafter, simply referred to as an xe2x80x9cignition coilxe2x80x9d) is a device for generating a spark in a gap of a spark plug by generating a high voltage through a mutual induction action. There are various types of ignition coils. For example, an ignition coil of a stick type that is adapted to be mounted in a plug hole has a rod-like core, a cylindrical secondary spool placed on an outer circumference side of the core, a secondary coil wound around the secondary spool, a cylindrical primary spool placed on an outer circumference side of the secondary coil, and a primary coil wound around the primary spool. Namely, the core, secondary spool, secondary coil, primary spool and primary coil are disposed concentrically, in that order, from the inside of the ignition coil. These constituent members are accommodated within a hollow cylindrical case. In addition, a resin insulating material is filled in the case in order to secure the electrical insulation properties of the respective constituent members accommodated within the case.
Thus, disposed within the interior of the case are components such as the primary and secondary coils which carry high voltages. On the other hand, disposed outside the case are a plug hole, a cylinder head and a vehicle frame. However, these members disposed outside the case carry relatively low voltage. Consequently, a base resin for forming the case needs to withstand a potential difference between the inside and outside of the case so that no electrical conduction is established therebetween. Conventionally, due to this, base resins for forming the case are required to have high electrical insulation properties. With a view to satisfying the requirement, polybutylene terephthalate (PBT), polyphenylene sulfide (PPS) and the like have been used as base resins for forming the case.
Incidentally, in recent years, there has been a strong demand for miniaturized ignition coils and, in particular, for those having smaller diameters. Here, making the case thinner can be taken as one of means for making ignition coils with small diameters. However, the electrical insulation properties are in proportion to the thickness of the case. Therefore, making the case thinner directly leads to a reduction in electrical insulation properties. Due to this, with a conventional case using PPS as the base resin, in the event that the thickness of the case is reduced, it is difficult to ensure the electrical insulation properties thereof and there is a risk that a dielectric breakdown occurs between the inside and outside of the case.
In addition, the case is prepared through resin molding. For example, in the event that a case is prepared through injection molding, molten resin, which is heated to be fluidized within a cylinder, is injected under a high pressure into a cavity in a mold and is then cooled to be set, whereby a case is prepared. Here, when attempting to mold a thin case, the width of portions of the mold cavity which correspond to case walls naturally becomes narrow. In order to allow the molten resin to be distributed to every corner of the interior of the narrow cavity, the base resin for forming the case needs to have high fluidity. In this respect, since PBT has a low fluidity, in the event that this resin is used as the base resin for the case, there is a risk that a defect such as a weld line may be generated. This then leads to a risk that a dielectric breakdown may occur at this defect portion, so that the case cannot ensure the desired electrical insulation properties.
Namely, PPS is insufficient in terms of the dielectric breakdown voltage performance, while PBT is insufficient in terms of fluidity. No resin has been found which can satisfy the both requirements.
An ignition coil according to the invention was made in view of the problem. An object of the invention is to provide an ignition coil of a reduced diameter having a case which is superior in electrical insulation properties and small in thickness.
With a view to attaining the object, according to the invention, there is provided an ignition coil having a case and a coil portion accommodated within the case, wherein the case comprises a base resin whose dielectric breakdown voltage exceeds that of polyphenylene sulfide and whose spiral flow length exceeds that of polybutylene terephthalate. In addition, more preferably, the case comprises a base resin whose dielectric breakdown voltage is equal to or exceeds that of polybutylene terephthalate and whose spiral flow length is equal to or exceeds that of polyphenylene sulfide.
Namely, according to the ignition coil of the invention, the case is formed from a base resin having both a dielectric breakdown voltage which exceeds that of PPS and a spiral flow length which exceeds that of PBT. More preferably, the case is formed from a base resin which has both a dielectric breakdown voltage which is equal to or exceeds that of PBT and a spiral flow length which is equal to or exceeds that of PPS.
Here, the dielectric breakdown voltage means a voltage at which the electric insulation of the case fails. The higher the dielectric breakdown voltage, the better the electrical insulation properties are. In addition, the spiral flow length means the overall length of the spiral of a molded article when the molded article whose configuration resembles a spiral mosquito-repellent incense is prepared by injecting a base resin in a molten condition into a spiral groove or the flow distance of the resin along the spiral groove. The longer the spiral flow length, the better the fluidity of the resin is.
The base resin for forming the case of the ignition coil of the invention has a long spiral flow length and a high fluidity. Owing to this, it is easy to mold a thin case. In addition, when molding, there is only a limited risk that a defect such as a weld line is caused. Furthermore, the base resin for forming the case of the ignition coil of the invention has a high dielectric breakdown voltage and good electrical insulation properties. Owing to this, even if the thickness of the case is reduced, there is only a limited risk that the insulation between the inside and outside of the case is broken down.
Here, the base resin may be such as to have both a dielectric breakdown voltage which exceeds that of PPS and a spiral flow length which exceeds that of PBT. Of course, it is more preferable that the base resin is such as to have both a dielectric breakdown voltage which is equal to or exceeds that of PBT and a spiral flow length which is equal to or exceeds that of PPS by improving both performances. Furthermore, the base resin preferably has, but is not limited to, a load-deflection temperature of 240 degrees C. or greater.
Here, the load-deflection temperature (the thermal deformation temperature) is measured, as is regulated in JIS (Japanese Industry Standard) K 7207-1983, by supporting a prismatic sample at two points in a heating bath and increasing the temperature of the bath while applying a predetermined bending stress at the center of the sample. In the measurement, a temperature at which the deflection of the sample reaches a predetermined amount is regarded as the load-deflection temperature. The higher the load-deflection temperature is, the higher the heat resistance of the resin is.
In many cases, the ignition coil is placed in a high-temperature environment such as in the vicinity of a cylinder. According to the construction of the invention, even in a case where the ignition coil is used in the high-temperature environment, there is only a limited risk that the case deforms due to heat.
According to the construction of the invention, while there is no particular limitation to kinds of base resins for use for the case, it is preferable to use, in particular, crystalline polystyrene (syndyotactic-polystyrene, SPS) as the base resin for the case.
Being different from a conventional non-crystalline polystyrene (PS), SPS has a construction in which benzene rings of side chains are coordinated alternately in opposite directions relative to main chains. Due to this construction, when compared to the conventional PS, SPS is largely improved in characteristics. SPS has a high dielectric breakdown voltage and a good fluidity, as well. Therefore, by using SPS as the base resin, it is possible to easily prepare a case which has a high dielectric breakdown voltage while being thin in thickness. In addition, SPS has a high load-deflection temperature. Owing to this, even in case where the ignition coil is disposed in a high-temperature environment, there is only a limited risk that the case deforms.
Furthermore, SPS has a property that a carbonized conductive track (track) is hardly formed even if the surface, where electrolysis occurs, is dirty with dust, dirt and moisture. Namely, SPS has high tracking resistance. In this respect, SPS is preferable as the base resin for forming the case of the ignition coil according to the invention.
In addition, while there is no particular limitation to places where the ignition coil is mounted, a construction is preferable in which the ignition coil is mounted in a plug hole in a cylinder. A so-called stick type ignition coil that is mounted in a plug hole is strongly demanded to have a reduced diameter. Owing to this, the ignition coil according to the invention which facilitates the reduction in thickness of the case is preferable for a stick type ignition coil.
In addition, with a view to solving the problem, according to the invention, there is provided an ignition coil having a case and a coil portion accommodated within the case, wherein the case is formed from a base resin whose dielectric breakdown voltage exceeds 15 kV/mm and whose spiral flow length exceeds 150 mm.
Here, the reason why the dielectric breakdown voltage is set to exceed 15 kV/mm is because there is a risk that the insulation of the case may be broken down, when the dielectric breakdown voltage is equal to or less than 150 kV/mm, when attempting to reduce the thickness of the case. In addition, the reason why the spiral flow length is set to exceed 150 mm is because the fluidity of the base resin becomes low with the spiral flow length being equal to or less than 150 mm and therefore it is difficult to form the case thin. In addition, with the spiral flow length being equal to or less than 150 mm, there is a risk that a defect, such as a weld line, may be caused during molding.
Preferably, the case may be formed from a base resin whose dielectric breakdown voltage is equal to or greater than 25 kV/mm and whose spiral flow length is equal to or longer than 170 mm. The reason why the dielectric breakdown voltage is set to be equal to or greater than 25 kV/mm is because the risk, that a dielectric breakdown is caused, becomes smaller with the dielectric breakdown voltage being equal to or greater than 25 kV/mm. In addition, the reason why the spiral flow length is set to be equal to or longer than 170 mm is because the fluidity of the resin becomes higher with a spiral flow length equal to or longer than 170 mm, whereby the reduction in thickness of the case is facilitated.
The invention may be understood more fully from the accompanying drawing and the following description of a preferred embodiment of the invention.